Vol.29 No.1/2 JOURNAL OF ELECTRONICS (CHINA) March 2012

DETECTION ON MICRO-DOPPLER EFFECT BASED ON LASER COHERENT RADAR1

Sun Yang Zhang Jun*

(Factory 704 of Navy, Qingdao 266109, China) *(Institute of Opto-electronic Information, Yantai University, Yantai 264005, China)

Abstract A laser coherent detection system of 1550 nm wavelength was presented, and experimen-tal research on detecting micro-Doppler effect in a dynamic target was developed. In the study, the return signal in the time domain is decomposed into a set of components in different wavelet scales by multi-resolution wavelet analysis, and the components are associated with the vibrational motions in a target. Then micro-Doppler signatures are extracted by applying the reconstruction. During the course of the final data processing frequency analysis and time-frequency analysis are applied to analyze the vibrational signals and estimate the motion parameters successfully. The experimental results indicate that the system can effectively detect micro-Doppler information in a moving target, and the tiny vibrational signatures also can be acquired effectively by wavelet multi-resolution analy-sis and time-frequency analysis.

Key words Micro-Doppler effect; Laser coherent radar; Multi-resolution analysis; Time-frequency analysis

CLC index TN958.98

DOI 10.1007/s11767-012-0544-1

I. Introduction Mechanical vibration or rotation of structures

in a radar target may induce frequency modula-tion on returned signals, and the modulation in-duced by vibrations or rotations is called micro- Doppler effect[1–3]. For example, mechanical vibra-tion of the engine inside of military vehicles or tanks[4], mechanical vibrations on the surface of the helicopter and rotor blade’s rotation, as well as human walking with swinging two arms[5], all of these have the characteristics of micro-Doppler signatures. Micro-Doppler signatures provide a specific target feature, and it suggests that appli-cations and development of micro-Doppler are potentially useful for target detection, classifica-tion, and recognition. Coherent laser radar has a good application prospects for its high precision measurement and strong anti-jamming ability[6–10]. Thus, in this study we establish the laser coherent detection system of 1550 nm wavelength to de-

1 Manuscript received date: August 12, 2010; revised date:

February 20, 2012. Communication author: Sun Yang, born in 1985, male, Master Degree. Factory 704 of Navy, Qingdao 266109, China. Email: sunyang6896@sina.com.

velop the experimental research on detecting mi-cro-Doppler effect in the dynamic target. From the analysis of experimental results, we can have the conclusion that the micro-Doppler signatures derived from the target’s vibration can be ex-tracted by utilizing multi-resolution analysis of wavelet transforms, and that the time-frequency analysis can reveal the signals characteristics fur-ther and estimate the motion parameters (the translation speed, the vibration’s amplitude and period). The classification and recognition of the target signtures are realized successfully.

II. Experimental Principle and System

1. The principle of laser micro-Doppler effect

In coherent radar, the phase of the target’s return signal is sensitive to variation in range. A half wavelength’s change in radial distance can cause 360° phase change. Thus, the Doppler fre-quency shift which represents the change of phase function with time, can be used to detect vibra-tions or rotations of structures in a target through the returns[9]. In the experiment, when the target is doing seesaw motion due to the movement of stepping motor, the phase change with time can cause the special Doppler frequency shift[11]. Then

SUN et al. Detection on Micro-Doppler Effect Based on Laser Coherent Radar 57

the tiny vibration is added. Via photodetector, the interference induced by the joint modulation of the translational and vibrational motions trans-lates into electrical signals, and the electrical sig-nals are transmitted in the digital storage oscillo-scope. The data collection and final processing are managed by computer.

Suppose the initial distance between target and detector is R0, the translational speed of tar-get is v. Simple harmonic vibration is added on the target, and the amplitude and frequency of vibration is D, ,ν respectively. The vibration can be described as:

( )sin 2tD D tπν= (1)

So the distance between target and detector at any time is

( )0 0( ) sin 2tR t R vt D R vt D tπν= + + = + + (2)

Then the received signal from detector can be denoted as:

[ ]{ }

( )( ) exp 2 4

exp 2 ( )

cc

c

R tS t j f t

j f t t

ρ π πλ

ρ π φ

⎧ ⎫⎡ ⎤⎪ ⎪⎪ ⎪⎢ ⎥= +⎨ ⎬⎢ ⎥⎪ ⎪⎪ ⎪⎣ ⎦⎩ ⎭= + (3)

where ρ is the reflectivity function, and ( )tφ = 4 ( )/ cR tπ λ is the phase function. Because the time-derivation of the phase is frequency, so take the time-derivation of phase, the Doppler fre-quency shift induced by translation and the mi-cro-Doppler frequency shift induced by vibration can be calculated as:

( )1 2 4

cos 22d

c c

d vf d t

dtφ π

ν πνπ λ λ

= = + (4)

where 2 / cv λ is the Doppler frequency shift, (4 /π ) cos(2 )c d tλ ν πν is the micro-Doppler frequency

shift. Thus translation speed, vibration amplitude, and vibration frequency have been associated with the Doppler frequency shift and micro-Doppler frequency shift. In other words, as long as we can acquire the Doppler frequency shift and micro- Doppler frequency shift, the translation and vi-bration signatures can be acquired.

2. Experimental system

The schematic drawing of micro-Doppler de-

tection system was shown in Fig. 1. The system operates based on coherent laser radar, and opti-cal fibers are used throughout the system from the laser to transmitter telescope and from re-ceiver telescope to the photodetector. The output power of the optical source is 6.15 mW (7.89 dBm) and the laser wavelength is 1550 nm. The laser beam from fiber laser is directed through a 10/90% coupler (C1). Then the output of 90% laser beam is launched into the transmitter tele-scope. The other output from the coupler C1 is directed to the photodetector via another coupler (C2), where it is combined with the 10% end of coupler C2, and the final output serves as the ref-erence light of interference. The diffused light from the dynamic target is collected by light col-lecting system, and is coupled into the receiver telescope. The collecting diffused light together with reference light generates the interference on the photodetector (DET01CFC), and then the output signals are transmitted in the digital stor-age oscilloscope (Tektronix TDS 3052). Through the GPIB interface the oscilloscope is connected with computer, and the data collection and final processing are managed by computer. In our ex-periment, stepping motor is employed as a tool of driving rotating platform and target, and makes them do low speed and uniform motion. Besides, vibrating object with controllable frequency and amplitude is used for making target simple har-monic vibration. In order to study the micro- Doppler signatures better, two groups of experi-mental data including translational motion (only translation) and vibrational motion (not only translation but also vibration) are chosen for comparison, and both of experimental conditions are the same.

Fig. 1 The schematic drawing of micro-Doppler effect detection system

58 JOURNAL OF ELECTRONICS (CHINA), Vol.29 No.1/2, March 2012

III. Experimental Results and Analysis

1. Experimental results

Fig. 2 gives the comparison of signal from translational target (shown in Fig. 2(a)) and vi-brational target (shown in Fig. 2(b)). The signal sampling time is 40 ms, and the data contains 1000 points. As can be seen from the Fig. 2(a), modulated by the translation at a constant speed, so the signal from translational target in the Fig. 2(a) has a basically constant amplitude and frequency. But the signal from vibrational target in the Fig. 2(b) demonstrates a variable-density according to the target vibration frequency. In the Fig. 2(b), the added vibration frequency is 100 Hz.

Fig. 2 Experimental data

2. Wavelet multi-resolution analysis

By wavelet transform, experimental[12] signal can be represented by a low frequency approxi-mation (smoothing information) and several high frequency details with different scales. The low fr-

equency approximation contains the average in-formation of the signal, and high frequency details contain the texture or edge feature of the signal. In order to make the micro-Doppler signatures separated from the returns, multi-resolution analysis of wavelet transforms is used to extract the micro-Doppler signatures[13,14]. Multi-resolution analysis was presented by Mallat in 1989[12], and the process of Mallat wavelet algorithm including decomposition (shown in Fig. 3(a)) and recon-struction (shown in Fig. 3(b)) is shown in Fig. 3,

Fig. 3 Mallat wavelet decomposition and reconstruction

where a1, a2, a3 are the smoothing information in different scales, and d1, d2, d3 are the high fre-quency details in different scales, respectively.

The return vibration signals in the time do-main is operated by wavelet decomposition and reconstruction at four scales, then respectively the smoothing informations (a1, a2, a3, a4) and high frequency details (d1, d2, d3, d4) are acquired at four scales, and the results are shown in Fig. 4(a) and Fig. 4(b).

After wavelet multi-resolution transform, we found that a2 in Fig. 4(a) has the better micro- Doppler signature. So the information of a2 is captured and shown in Fig. 5(a). Based on the transform and analysis upwards, we can also ac-quire the better translational information of tar-get including doppler signature, and the result is given in Fig. 5(b).

3. Frequency domain analysis

Doppler and micro-Doppler frequency shift can be seen from frequency domain, so Fast Fourier Transform (FFT) is used for extracting the micro- Doppler characteristics from the acquired signal[5]. Fig. 6(a) and Fig. 6(c) are the FFT results of the signals in Fig. 5(a) and Fig. 5(b), respectively. Fig. 6(a) and Fig. 6(c) show the signal spectrum of vibrational target and translational target.

SUN et al. Detection on Micro-Doppler Effect Based on Laser Coherent Radar 59

Fig. 4 The vibration signal at four scales by wavelet multi-reso-lution transform

Fig. 5 The low frequency approximation at second scales, which contains better signature

From the figures we found that there is zero- Doppler component disturbing the signal spec-trum. So the mean method is utilized for reducing and eliminating the zero-Doppler. After the origi-nal signal data minus the mean, the FFT is re-peated and the results are given in Fig. 6(b) and Fig. 6(d). Apparently, we reduce the disturbing effect of zero-Doppler, and the characteristics are represented clearly by the spectrum. The spec-trum of translational target (shown in Fig. 6(d)) only contains one peak in each side of the axis, where the peaks at positive and negative axis are symmetrical. The position of the peak reflects the Doppler frequency shift induced by translation. It implied that this signal contains a single fre-quency component, and uniform translational speed makes the interference vary constantly. Fig. 6(b) shows that the signal spectrum of vibra-tional target has abundant frequency components, and frequency components are expanded with the frequency band center approximately at the posi-tion of the peak in Fig. 6(d). Due to vibration and translation of target, it may induce the frequency modulation which generate sidebands about the target’s Doppler frequency. The spectrum can show which frequency components is contained in the signal, but it is hard to know when a certain frequency occurs and how the signal frequency varies with time. It is a vital defect for the analy-sis of time-varying micro-Doppler signal. In order to solve this issue, a joint time-frequency analysis will be introduced to analyze the micro-Doppler signal.

60 JOURNAL OF ELECTRONICS (CHINA), Vol.29 No.1/2, March 2012

Fig. 6 Spectrum

4. Time-frequency analysis

There are many methods in time-frequency analysis, but all the methods have severally proc-essing objects and effect. After the comparison and analysis of abundant micro-Doppler experi-mental data, we introduce the transmutation of Wigner-Ville Distribution (WVD)-Smoothed Pseudo WVD (SPWVD)[5], which has the better effect in crosscomponents suppression. Micro- Doppler characteristics can be well extracted from the SPWVD, so we use it to process and analysis acquired signals.

Fig. 7(a) shows time-frequency signal charac-teristics from translational target. It indicates that this signal contains a single frequency com-ponent, and the frequency does not change with time. This conclusion is the same to the analysis result in frequency domain. The single frequency reflects the Doppler frequency shift induced by translation, and the translation speed of the tar-get can be calculated by Eq. (4). From the Fig. 7(a), we can estimate that the frequency is about 850 Hz. Therefore, by calculation the trans-lational speed is about 0.66 mm/s. The relation-ship of frequency change with time is clearly il-lustrated in Fig. 7(b), and the relationship is co-sine curve approximately.

From Fig. 7(b), we found that the period of frequency change is about 10 ms. It corresponds to the vibrational period of target, so the vibra-tional period of target is about 10 ms, which is according with the set frequency of 100 Hz. The frequency band width can also be seen from the time-frequency distribution, and it is estimated about 1 kHz. Then the amplitude of vibration can be derived by Eq. (4) as about 0.62 μm.

IV. Conclusion We have developed the laser coherent detec-

tion system of 1550 nm wavelength for micro- Doppler effect study. Wavelet multi-resolution analysis was utilized for extracting the micro- Doppler signatures in order to separate micro- Doppler signatures from the return signals. Then these signals including micro-Doppler information were analyzed in frequency domain and time- frequency domain. The characteristics of micro- Doppler signatures were revealed better by the

SUN et al. Detection on Micro-Doppler Effect Based on Laser Coherent Radar 61

time-frequency method of SPWVD. The experi-ment proves that this system can detect micro- Doppler signatures successfully, and time-fre-quency distribution is availably used for analysing micro-Doppler signatures. This work provides an effective way for target detection, classification, and recognition.

Fig. 7 Time-frequency characteristics of the signal from different targets

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